Gas quenching cell for steel parts

ABSTRACT

A method for quenching a steel load by flowing of a gas at the load level via gas moving means. The moving means are controlled to have the gas flow at the load level at a speed which varies according to a speed profile, at least a portion of which comprises, successively, a plateau at a first speed and a plateau at a second speed greater than the first speed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas-quenching cell for steel parts and more specifically a steel part gas quenching method implemented in such a quenching cell.

2. Discussion of the Related Art

Steel part gas quenching methods have many advantages over liquid quenching methods, especially the fact that the processed parts come out dry and clean.

The gas quenching of steel parts which have previously undergone a heat processing (heating before quenching, anneal, tempering . . . ) is generally performed with a pressurized gas, generally between 4 and 20 bars. The quenching gas for example is nitrogen, argon, helium, carbon dioxide, or a mixture of these gases.

A quenching operation consists of rapidly cooling down steel parts which generally are at temperatures ranging between 750° C. and 1000° C. At such temperatures, the steel essentially is in the form of austenite, which is only stable at high temperatures. A quenching operation provides by a fast cooling down, a transformation of the austenite into martensite, which exhibits high hardness properties. The quenching operation must be relatively fast so that all the austenite turns into martensite with no forming of other steel phases of perlite or bainite type, which have lower hardness properties than martensite.

A quenching cell generally comprises at least one motor, generally of electric type, rotating a stirring element, for example, a helix, adapted to circulating the quenching gas in the quenching cell. To obtain a fast cooling down of the parts introduced into the quenching cell, the quenching gas is usually circulated at the level of the parts to be cooled down at the highest possible speed for the entire quenching operation.

A quenching operation is thus conventionally performed by imposing a static quenching gas pressure in the quenching cell, and by controlling the motor at a maximum rotation speed to obtain a maximum circulation speed of the quenching gas at the level of the steel parts to be cooled down.

Although the previously-described gas quenching methods enable obtaining quenched parts having a quite satisfactory fatigue strength, it would be desirable to provide a gas quenching method enabling further improving the fatigue strength of the quenched parts.

Further, although the previously-described gas quenching methods enable obtaining quenched parts having strongly decreased deformations with respect to oil quenching methods, it would be desirable to provide a gas quenching method enabling further decreasing the deformations of the quenched parts.

SUMMARY OF THE INVENTION

The present invention aims at obtaining a method for quenching steel parts and a quenching cell for implementing such a method providing quenched parts with an improved fatigue strength and/or with reduced deformations.

Another object of the present invention is to obtain a quenching cell enabling implementation of the quenching method according to the present invention and with a structure which is little modified with respect to a conventional quenching cell.

For this purpose, the present invention provides a method for quenching a steel load by flowing of a gas at the level of the load via gas moving means. The moving means are controlled to have the gas flow at the load level at a speed which varies according to a speed profile, at least a portion of which comprises, successively, a plateau at a first speed and a plateau at a second speed greater than the first speed.

According to an embodiment of the present invention, the gas, after having flowed at the level of the load, is cooled down by an exchanger in which flows a cooling fluid. The moving means are controlled to have the gas flow at the level of the load from the plateau at the first speed to the plateau at the second speed when the temperature of the cooling fluid reaches a given threshold temperature.

According to an embodiment of the present invention, the static pressure of the gas at the level of the load is decreased during the plateau at the first speed with respect to the plateau at the second speed.

According to an embodiment of the present invention, the gas, after having flowed at the level of the load, is cooled down by an exchanger in which flows a cooling fluid, the moving means being controlled to have the gas flow at the load level according to a speed profile successively comprising a first plateau at the second speed, a plateau at the first speed and a second plateau at the second speed, the transition between the first plateau at the second speed and the plateau at the first speed being performed during a cooling fluid temperature increase phase.

According to an embodiment of the present invention, the moving means are controlled to have the gas flow at the level of the load from the first plateau at the second speed to the plateau at the first speed when the cooling fluid temperature exceeds a given threshold temperature.

According to an embodiment of the present invention, the moving means are controlled to have the gas flow at the level of the load from the plateau at the first speed to the second plateau at the second speed when the cooling fluid temperature decreases under a given additional threshold temperature.

According to an embodiment of the present invention, the moving means are controlled to have the gas flow at the level of the load from the first plateau at the second speed to the plateau at the first speed after a determined time.

According to an embodiment of the present invention, the gas, after having flowed at the level of the load, is cooled down by a exchanger in which flows a cooling fluid, the moving means being controlled to have the gas flow at the level of the load according to a speed profile comprising, from the beginning of a quenching operation, successively a plateau at the first speed and a plateau at the second speed, the transition between the plateau at the first speed and the plateau at the second speed being performed during an increase phase of the cooling fluid temperature.

According to an embodiment of the present invention, the moving means are controlled to have the gas flow at the level of the load from the plateau at the first speed to the plateau at the second speed after a determined time.

The present invention also provides a gas quenching cell of a load comprising a stirring element driven by a motor to cause a gas flow between the load and an exchanger. The cell comprises means capable of varying the stirring element drive speed to have the gas flow at the load level at a speed which varies according to a speed profile comprising at least successively a plateau at a first speed and a plateau at a second speed greater than the first speed.

The foregoing objects, features, and advantages of the present invention, as well as others, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show two views of an example of the forming of a gas quenching cell according to the present invention;

FIG. 2 shows the variation of the quenching gas speed at the level of a load contained in a quenching cell according to FIGS. 1A and 1B and the temperature variation of the cooling fluid of an exchanger of the cell in the case of a conventional quenching method;

FIG. 3 shows the variation of the quenching gas speed at the level of a load contained in a quenching cell according to FIGS. 1A and 1B and the temperature variation of the cooling fluid of an exchanger of the cell in the case of a first example of a quenching method according to the present invention;

FIG. 4 shows the variation of the temperature at the level of a load contained in a quenching cell according to FIGS. 1A and 1B processed according to a conventional quenching method and the first example of quenching method according to the present invention;

FIG. 5 shows the variation of the quenching gas speed at the level of a load contained in a quenching cell according to FIGS. 1A and 1B and the temperature variation of the cooling fluid of an exchanger of the cell in the case of a second example of a quenching method according to the present invention; and

FIG. 6 shows the variation of the quenching gas speed at the level of a load contained in a quenching cell according to FIGS. 1A and 1B and the temperature variation of the cooling fluid of an exchanger of the cell in the case of a third example of a quenching method according to the present invention.

DETAILED DESCRIPTION

FIGS. 1A and 1B schematically show a lateral cross-section view and a front cross-section view of a gas quenching cell likely to be used according to the present invention. The cell comprises an enclosure 10 of general cylindrical or parallelepipedal shape with a horizontal axis. The cell is closed at one end while the other end comprises a guillotine door system 12 providing access to the cell to introduce into it or extract from it a load to be processed 14. Of course, door 12 enables tightly closing the quenching cell. Load 14 is maintained substantially at the center of the cell on a plate 16.

The upper cell portion is provided with two external motors with a vertical axis 18, arranged next to each other in the longitudinal cell direction. Such motors drive respective stirring elements 20 inside of the cell. As an example, motors 18 are electric motors.

As visible in FIG. 1B, the cell is provided with an exchanger 22 arranged on either side of load 14 in a horizontal plane. Exchanger 22 comprises a cooling fluid circulation duct and is capable of cooling the quenching gas flowing therethrough. Between exchanger 22 and load 14 are arranged guiding plates 24 which join stirring devices 20 to direct the gas flow generated by the latter between load 14 and exchanger 22. With this configuration, the quenching gas flows, for example, downwards through load 14 and upwards through exchanger 22. As an example, stirring elements 20 are turbines or ventilators. The quenching gas is, for example, nitrogen or a mixture of carbon dioxide and helium.

The present invention consists of controllably modifying the quenching gas circulation speed at the level of load 14 in a quenching operation. For this purpose, quenching cell 18 is equipped with a speed variation system. As an example, the speed variation may be obtained via a frequency variator for electric motors. In the case where motors 18 are hydraulic motors, a system of variation of the oil flow supplying motors 18 may be provided.

According to the present invention, the elaboration of a speed profile of the quenching gas flowing at the level of load 14 is obtained from a characteristic parameter representative of the average temperature at the level of load 14. The characteristic parameter corresponds to the cooling fluid temperature at the level of the exit of exchanger 22, that is, when the temperature of the cooling fluid flowing through exchanger 22 is highest. Indeed, the curve representative of the cooling fluid temperature variation at the exit of exchanger 22 is characteristic of the power taken from load 14.

FIG. 2 illustrates the principle underlying the selection of the cooling fluid temperature at the exit of exchanger 22 as a characteristic parameter to vary the quenching gas circulation speed. FIG. 2 shows a conventional embodiment of a curve 26 of variation of the quenching gas speed at the level of load 14, in which the quenching gas flow speed is constant and corresponds to the maximum of the capacities of motors 18. FIG. 2 also shows a curve 30 of the variation of the quenching fluid temperature at the exit of exchanger 22 obtained for such a speed profile. Curve 30 comprises an ascending portion 32 deviating at the level of a highest point 34 and followed by a descending portion 36.

The applicant has shown that the austenite-martensite transformation of the steel forming load 14 substantially occurs at the level of highest point 34 of curve 30. The applicant has shown that an improvement of the fatigue strength may be obtained by limiting the temperature variations of load 14 in the austenite-to-martensite transformation to enable the austenite-to-martensite transition to occur at relatively homogenous temperatures of load 14.

FIG. 3 shows a curve 40 representative of the variation of the quenching gas flow speed at the level of load 14 for a first example of a quenching method according to the present invention and a curve 42 representative of the temperature variation of the cooling fluid of exchanger 22 corresponding to such a quenching gas speed profile. As a comparison, curve 30 of variation of the cooling fluid temperature for a quenching gas flowing at maximum speed for the entire quenching operation has been reproduced in dotted lines.

The first quenching method according to the present invention consists of controlling motors 18 so that the quenching gas flow speed at the level of load 14 successively corresponds to a first maximum speed plateau 42 for a time T1, to an intermediary speed plateau 44 for a time T2, and to a second maximum speed plateau 46 until the end of the quenching operation. As an example, during plateau 44, motors 18 are controlled so that the quenching gas flow speed drops by 30 to 60% with respect to the maximum speed. During first plateau 42, curve 42 of variation of the cooling fluid temperature comprises an ascending portion 48 which substantially follows that of curve 30. In the intermediary speed plateau 44, the cooling fluid temperature tends to stabilize so that curve 40 comprises a portion of small variations 50. In the second maximum speed plateau 46, curve 42 follows a descending portion 52.

The transition from first maximum speed plateau 42 to intermediary speed plateau 44 is performed when the cooling fluid temperature reaches a first given threshold temperature, which corresponds to a temperature slightly lower than the temperature at highest point 34 of curve 30. It thus substantially is the cooling fluid temperature for which the austenite-to-martensite transformation of load 14 starts. The transition from intermediary speed plateau 44 to second maximum speed plateau 46 is performed when the cooling fluid temperature, towards the end of low variation portion 50, decreases beyond a second given threshold temperature, for example, equal to the first given threshold temperature, and which is representative of the fact that the austenite-to-martensite transformation of load 14 is over.

The austenite-to-martensite transformation of load 14 is then totally performed for a quenching gas flow speed smaller than the maximum value. Advantageously, the intermediary speed is adjusted to a value such that the thermal power recovered by exchanger 22 corresponds to the thermal power released by load 14 during the austenite-to-martensite transformation, which is an exothermic reaction. The temperature of load 14 is then maintained at a substantially constant and homogenous temperature during the entire austenite-to-martensite transformation of the entire load 14. In practice, the intermediary speed is adapted to obtain as constant as possible a temperature of the cooling fluid during portion 50.

In the first example of embodiment, the static pressure of the quenching gas may be maintained at a constant value for the entire quenching operation, between 4 and 20 bars. According to an alternative of the first embodiment, the static pressure of the quenching gas in the quenching cell is decreased on application of the intermediary speed plateau in a range from 30% to 80% of the static quenching gas pressure during the first and second maximum speed plateaus. This enables controlling, in combination with the intermediary quenching gas speed, the thermal power recovered at load 14 during the austenite-to-martensite transformation.

FIG. 4 shows two curves 54, 56 of variation of the temperature measured at the level of load 14 during a quenching operation of load 14 respectively for a conventional quenching method for which the quenching gas flow speed remains constant and maximum and the first example of a quenching method according to the present invention. More specifically, curve 56 has been obtained in the case where duration T1 of application of first maximum speed plateau 42 is of 50 seconds and duration T2 of intermediary speed plateau 44 is 310 seconds. The intermediary speed corresponds, in the present example, to 30% of the maximum speed. The static pressure of the quenching gas, which is, in the present example, nitrogen, is 16 bars during the first and second maximum speed plateaus 42, 46, and 2 bars during intermediary speed plateau 44. It should be noted that after 50 seconds, curve 56 decreases much less than curve 54. The temperature variation of load 14 is thus limited during the austenite-to-martensite transformation.

The applicant has shown an improvement of the fatigue strength of the parts forming load 14 quenched according to the first example of quenching method of the present invention. An explanation would be that since the austenite-to-martensite transformation occurs at temperatures with limited variations, much less internal mechanical stress appears in load 14, whereby the fatigue strength is improved.

As an example, for a load 14 formed of a 27MnCr5-type steel and processed according to a low-pressure cementation method, the applicant has shown an increase in the fatigue strength on the order of 20% with respect to a cold oil quenching (oil at 60° C.) or a nitrogen quenching at constant pressure (16 bars) and at maximum quenching gas flow speed.

The first and second threshold temperatures depend on many parameters, especially on the type of steel forming load 14 and on the area of the exchange surface between load 14 and the quenching gas. The first and second threshold temperatures may be determined by quenching load 14 with a maximum gas flow speed to determine curve 30 shown in FIG. 2 associated with load 14. The first and second threshold temperatures then correspond to a given percentage of the maximum temperature of curve 30. The first example of the method of the present invention can then be implemented for a same load type by providing a temperature sensor at the level of the exit of exchanger 22 connected to a microcontroller capable of controlling motors 18. The transitions from first maximum speed plateau 42 to intermediary speed plateau 44 and from intermediary speed plateau 44 to second maximum speed plateau 46 are respectively performed when the cooling fluid temperature exceeds the first threshold temperature and decreases below the second threshold temperature. According to another variation, from curve 30, time T1 required for the cooling fluid temperature to reach the first threshold temperature can be determined. It is then not necessary, in normal operation, to provide a temperature sensor at the level of exchanger 22, the transition from first maximum speed plateau 42 to intermediary speed plateau 44 being automatically performed at the end of time T1. The transition from intermediary speed plateau 44 to second maximum speed plateau 46 can then be automatically performed at the end of time T2, determined, for example, empirically.

The present invention also provides a second example of a quenching method of a load 14 enabling reducing the deformations of load 14 during the quenching operation, especially local deformations of the load when said load comprises parts of complex shapes. This enables limiting the subsequent rectification steps to be provided for the quenched parts and/or simplifying the previous steps of design of the part shapes before quenching.

FIG. 5 shows a curve 58 representative of the variation of the quenching gas flow speed at the level of load 14 for the second example of quenching method according to the present invention and a curve 60 representative of the temperature variation of the cooling fluid of exchanger 22 obtained with such a quenching gas speed profile. As a comparison, curve 30 of variation of the quenching fluid temperature has been reproduced for a quenching gas flowing at maximum speed for the entire quenching operation.

The second embodiment of the quenching method of the present invention consists of controlling motors 18 so that the quenching gas flow speed at the level of load 14 successively corresponds to a first intermediary speed plateau 62 for a duration T1′ and to a second maximum speed plateau 64 on to the end of the quenching operation. As an example, during intermediary speed plateau 62, motors 18 are controlled so that the quenching gas speed varies between 0% and 70% of the maximum speed. During plateau 62, curve 60 of variation of the cooling fluid temperature comprises an ascending portion 66 less marked than ascending portion 32 of curve 30. The cooling fluid temperature thus increases slower than in the case where the quenching speed is maximum. In maximum speed plateau 64, ascending portion 66 carries on up to a highest point 68 and continues in a descending portion 70. Time T1′ may extend from 5 to 30 seconds according to the total duration of the quenching operation. Further, duration T1′ may be determined empirically.

During time T1′, the cooling speed of load 14 is smaller than that which would result from a maximum quenching gas flow speed. Since the cooling is slower, the deformations of load 14 are less significant. At the end of time T1′, since the load has cooled down, the mechanical inertia of load 14 has increased. Such an increase in the mechanical inertia limits subsequent deformations of load 14 when the quenching gas flow speed increases back again. The local deformations of load 14, in the quenching operation, are thus globally reduced since the cooling of load 14 with the maximum quenching gas flow speed is performed when the load has already acquired a sufficient mechanical inertia and thus opposes a resistance greater than the deformations.

In the second example of embodiment, the static pressure of the quenching gas may be maintained constant for the entire quenching operation. According to an alternative, in the transition from intermediary speed plateau 62 to maximum speed plateau 64, an increase in the static pressure of the quenching gas may be provided. The static pressure may be increased by from 2 to 5 times the initial pressure to reach a value, for example, between 4 and 20 bars.

As an example, for a load 14 comprising wheels with helical teeth formed of a 15CrM6 type steel, the applicant has shown a reduction of the deformations at the level of the tooth profile in a plane perpendicular to the helix direction, likely to reach approximately 45% with respect to a hot oil quenching (oil at 180° C.) and approximately 30% with respect to a gas quenching at maximum quenching gas flow speed.

The present invention also provides a third example of a method for quenching a load 14 corresponding to the combination of the two previously-described examples of embodiment. The third example of embodiment thus enables obtaining an improvement of the fatigue strength of the parts forming the load and a reduction in the deformations of the parts forming load 14.

FIG. 6 shows a curve 72 representative of the variation of the quenching gas flow speed at the level of load 14 for the third example of a quenching method according to the present invention and a curve 74 representative of the temperature variation of the cooling fluid of exchanger 22 obtained with such a quenching gas speed profile. As a comparison, curve 30 of variation of the cooling fluid temperature has been shown in dotted lines for a quenching gas flowing at maximum speed for the entire quenching operation.

The third example of embodiment of the quenching method of the present invention consists of controlling motors 18 so that the quenching gas flow speed at the level of load 14 successively corresponds to an intermediary speed plateau 76 for a time T1″, a maximum speed plateau 78 for a time T2″, an intermediary speed plateau 80 for a time T3″, and a maximum speed plateau 82 until the end of the quenching operation. As an example, during intermediary speed plateau 76, motors 18 are controlled so that the quenching gas flow speed varies between 0% and 70% of the maximum speed and that, during intermediary speed plateau 80, the quenching gas flow speed varies between 40% and 70% of the maximum speed.

In plateau 76, curve 74 of the variation of the cooling fluid temperature comprises an ascending portion 84 less marked than ascending portion 32 of curve 30. During maximum speed plateau 78, curve 74 comprises an ascending portion 86 more strongly marked than ascending portion 84. In intermediary speed plateau 80, curve 74 comprises a small variation plateau 88 and during maximum speed plateau 82, curve 74 comprises a descending portion 90.

Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the quenching cell may be different from the previously-described cell. In particular, the axis of motors 18 may be arranged horizontally, the quenching gas flow at the level of load 14 occurring substantially horizontally. Further, the cell may comprise a duct forming a loop outside of the cell, exchanger 22 being inserted in the duct.

Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto. 

1. A method for quenching a steel load (14) by flowing of a gas at the level of the load via gas moving means (18, 20), wherein the moving means are controlled to have the gas flow at the load level at a speed which varies according to a speed profile, at least a portion of which comprises, successively, a plateau at a first speed (44; 62) and a plateau at a second speed (46; 64) greater than the first speed.
 2. The method of claim 1, wherein the gas, after having flowed at the level of the load (14), is cooled down by a exchanger (22) in which flows a cooling fluid, the moving means being controlled to have the gas flow at the level of the load (14) from the plateau at the first speed (44; 62) to the plateau at the second speed (46; 64) when the temperature of the cooling fluid reaches a given threshold temperature.
 3. The method of claim 1, wherein the static pressure of the gas at the level of the load (14) is decreased during the plateau at the first speed (46; 64) with respect to the plateau at the second speed (44; 62).
 4. The method of claim 1, wherein the gas, after having flowed at the level of the load (14), is cooled down by an exchanger (22) in which flows a cooling fluid, the moving means (18, 20) being controlled to have the gas flow at the load level according to a speed profile successively comprising a first plateau at the second speed (42), a plateau at the first speed (44) and a second plateau at the second speed, the transition between the first plateau at the second speed (46) and the plateau at the first speed being performed during an increase phase of the cooling fluid temperature.
 5. The method of claim 4, wherein the moving means are controlled to have the gas flow at the level of the load (14) from the first plateau at the second speed (42) to the plateau at the first speed (44) when the cooling fluid temperature exceeds a given threshold temperature.
 6. The method of claim 4, wherein the moving means are controlled to have the gas flow at the level of the load (14) from the plateau at the first speed (44) to the second plateau at the second speed (46) when the cooling fluid temperature decreases under a given additional threshold temperature.
 7. The method of claim 4, wherein the moving means are controlled to have the gas flow at the level of the load (14) from the first plateau at the second speed (42) to the plateau at the first speed (44) after a determined time.
 8. The method of claim 1, wherein the gas, after having flowed at the level of the load (14), is cooled down by a exchanger (22) in which flows a cooling fluid, the moving means (18, 20) being controlled to have the gas flow at the load level according to a speed profile comprising, from the beginning of a quenching operation, successively a plateau at the first speed (62) and a plateau at the second speed (64), the transition between the plateau at the first speed and the plateau at the second speed being performed during an increase phase of the cooling fluid temperature.
 9. The method of claim 8, wherein the moving means are controlled to have the gas flow at the level of the load (14) from the plateau at the first speed (62) to the plateau at the second speed (64) after a determined time.
 10. A gas quenching cell of a load (14) comprising a stirring element (20) driven by a motor (18) to cause a gas flow between the load and an exchanger (22), comprising means capable of varying the stirring element drive speed to have the gas flow at the load level at a speed which varies according to a speed profile comprising at least successively a plateau at a first speed (44; 62) and a plateau at a second speed (46; 64) greater than the first speed. 